For me all four bikes look good enough on paper.
There are some differences, which may (may, not must!) become important in the long run.
The Boardman SLR 9 has a proprietary seatpost shape and a proprietary seatpost clamp. That may lead to problems with spare parts availability and creaking. The cables for the front and rear derailleurs and the brake hoses are routed fully internal. If you want to service the headset, you have to more or less dismantle the brakes and the gearing, because the cables and hoses are routed through the upper headset bearing. That might become an issue if you plan to switch to a shorter or longer stem.
The Boardman SLR 8.9 has a proprietary seatpost clamp. That may lead to problems with spare parts availability and creaking.
On the Trek Domane AL 4 the cables for the front and rear derailleurs and the brake hoses are routed fully internal. If you want to service the headset, you have to more or less dismantle the brakes and the gearing, because the cables and hoses are routed through the upper headset bearing. That might become an issue if you plan to switch to a shorter or longer stem.
The Specialized Allez E5 Sport has a chainset with a standard 5 hole 110mm BCD. This may be an advantage if you want to try out different chain ring sizes, because there are more options available than with the proprietary Shimano cranks.
If you plan to do the maintenance of your bike yourself, then the fully internal routing may become an annoyance. If you plan to give the bike to a LBS for maintenance, then it probably will be more expensive when the headset bearings must be changed.
If not done yet: Please do a test ride on a bike with the Tiagra brifters and also on a bike with the 105 brifters. The hand position on the hoods is one of the most used position on road bikes. I've got a bike with Tiagra brifters and couldn't find a cormfortable position for my hands on the hoods. (*) Your hands may vary. It is doable, but in most cases it is non-economic to change the Tiagra brifters to another model, because this Tiagra is 10 speed and afaik not compatible to 105 or other Shimano group sets. If the Tiagra brifters are comfortable for you - very good. Same is for the 105 brifters.
You probably already know that the saddle is also a very personal thing and that other than the included saddle might be more comfortable.
None of the four bikes is a bad bike. Imho they're all more than good enough.
E.
(*) Because I don't like the Shimano control concept, my plan from the beginning was and is to sell the Tiagra group set and replace it with Campagnolo Veloce. So the - for me - uncomfortable shape of the Shimano brifters showed me that my approach is still the right one - for me.
